On the second day of incubation a blood ring appears
around the area opaca. The question mark shape in
the middle of the ring is the embryo. By 24 hours,
pockets of blood cells and developing blood vessels,
called blood islands, begin to appear on the surface
of the yolk along the outer edges of the area
opaca. These are called the vitelline vessels. At
the same time, the heart and vessels within the
chick embryo have begun to develop. The
omphalomesenteric veins can be seen branching out
from the embryo in this photo of a 43 hour chick
embryo. The vitelline vessels and the branches of
the omphalomesenteric arteries and veins grow toward
each other. When they meet, channels open and
circulation from the yolk to the embryo
begins. Vitelline circulation is responsible for
bringing food from the yolk to the embryo.